High resolution ultrasonography utilizes appropriate low energy sound waves to assess the sound transmission and reflective qualities of human tissue. Ultrasound images are formed by means of echoes among tissues with different acoustic impedance. Acoustic impedance is the product of sound speed and bulk modulus. The bulk modulus expresses the elasticity of an object, and in the human body the value is increased by conditions such as fibrosis and calcification. The sound speed is usually high in elastic tissues and low in water. In the body, it is lowest in the fatty tissues. When ultrasound passes through tissue, it is absorbed as thermal energy and attenuated. When the attenuation rate increases, the posterior echoes are attenuated. However, in tissues with a high water content, such as cysts, the posterior echoes are accentuated.
The machines used for ultrasound examination include a hand-held transducer which houses a crystal that is vibrated electronically. The transducer is placed on the surface of the patient's skin, and the ultrasound produced by the vibrating crystal penetrates the underlying tissue. The returning echoes are displayed as a digital image that represents the intensity of the ultrasound echoed or reflected by the underlying tissue. The tissue is examined in a series of very thin vertical sections or slices, and the depth of any particular tissue element within a section is determined according to the time it takes an echo reflected from that element to return to the transducer.
Modern ultrasound machines have a feature that is referred to as "real time". This consists of a linear array of separate transducers which are activated sequentially and which transmit successive waves of sound. This feature permits the detection of motion of anatomic parts, such as the expansile pulsation of blood. vessels. In obstetrical applications, this ability to detect motion permits recognition of early fetal heart activity and, later, movement of fetal extremities.
In performing an ultrasound examination, the technologist first applies a thick layer of an aqueous gel to the surface of the skin above the tissue area of interest. It is a well-known to those skilled in the art of ultrasonography that while water transmits sound very well, most ultrasound detail lying beneath an air layer or pocket will be obscured. The gel replaces the air at the skin's surface and allows a direct fluid connection between the transducer and the skin.
After the gel has been applied to the area of interest, the technologist moves the transducer through the gel while studying the images on a video monitor. The images are also stored on a permanent recording medium, such as film, a laser print or a photograph. It is important to recognize that, currently, ultrasound examinations are conducted without reference to any consistent, fixed, reproducible or definable landmarks. Only the technologist knows the position and orientation of the transducer when an image is recorded. There are of course standard views, and a serious attempt is made to do transverse and longitudinal scans in some order, but ultrasonagraphy is basically a free hand study.
The surface position of the ultrasound transducer is, by custom, recorded in hand written notes during or after the image is made. Accordingly, the surface position of the transducer is described in a general relationship to well-known or visible surface anatomy or to other descriptive clinical findings. (E.g., "above and lateral to navel" or "below and lateral to palpable mass"). In state of the art equipment, this general information may be inscribed electronically onto the image with accompanying brief references during or after the examination. Later, when the film from the examination is being interpreted, the interpreter must accept the technologist's handwritten and/or electronic notes regarding the position of the transducer as factual. However, in the event that the area being examined is of clinical interest, there is no clear visible evidence included in the image indicating that the transducer is positioned over this area.
It is, therefore, an object of the invention to provide a surface marker for use in ultrasonography which provides a distinct image on a video monitor or permanent recording media which locates the position of the transducer with respect to the position of the marker on the surface of the tissue being examined.
It is yet another object of the invention to provide a such a marker which does not obscure underlying ultrasound tissue detail.
It is still another object of the invention to provide a marker that will conform to the surface of the anatomical part to which it is applied and which will remain fixed in place during the ultrasound examination.